Hitherto, as packaging materials to be sterilized by hydrothermal treatments, there have been frequently used laminates prepared by bonding an aluminum foil and a film with each other, and deposited films prepared by vapor-depositing aluminum on a surface of the film. However, since such packaging materials are opaque due to aluminum used therein, there is caused such a problem that resultant packages filled with contents are no longer subjected to inspections for metal foreign materials in the contents and appearance of the contents.
Consequently, in recent years, there have been proposed many high-transparency gas-barrier films that are prepared by forming a metal oxide thin film comprising silicon oxide, aluminum oxide or magnesium oxide which has a thickness of 10 to 100 nm, on the surface of a base plastic film. These gas-barrier films are usually in the form of a laminate further including a separate plastic film layer laminated on the metal oxide thin film, and are used as packaging materials for foods, medicines or the like.
It has been expected that the above gas-barrier laminates are used as retort packaging materials. However, in such application fields, the packaging materials have been required to still maintain the gas-barrier property even after being subjected to hydrothermal treatments.
However, the metal oxide thin film formed in the above laminates tend to be readily broken upon the hydrothermal treatments, resulting in remarkable deterioration in gas-barrier property thereof. The gas-barrier property of the laminates may be maintained by increasing the thickness of the metal oxide thin film, for example, up to about 50 nm or more. In this case, however, the productivity of the laminates tends to be deteriorated, resulting in increased production costs. Further, there may arise problems such as poor transparency of the obtained laminates.
On the other hand, there have been proposed the method of subjecting a deposited film including a metal oxide thin film to heat treatment as a post treatment thereof for a short period of time, e.g., for a period of from about several seconds to about one minute in order to enhance an adhesion between the metal oxide thin film and the other plastic film layer (Japanese Patent Application Laid-open (KOKAI) No. 55-84332(1980)), and the method of forcibly irradiating a deposited film with a visible light for a long period of time to enhance a transparency of the film and stabilize a gas-barrier property thereof (Japanese Patent Application Laid-open (KOKAI) No. 8-197674). However, these conventional methods have failed to attain the effect of improving a gas-barrier property of the gas-barrier films having the metal oxide thin film after the hydrothermal treatments.
On the other hand, there have been proposed the method of successively subjecting a deposited film to water or moisture absorption treatment and then heat treatment in order to maintain a transparency of the film and improve a gas-barrier property thereof (Japanese Patent Application Laid-open (KOKAI) No. 2-299826), and the method of heat-treating a laminated film including a biaxially stretched film having a metal oxide thin film, and the other film laminated on the metal oxide thin film through an adhesive resin layer, for a short period of time at a temperature of from a second-order transition point to a melting point of the biaxially stretched film, more specifically at about 200° C. at which the adhesive resin layer can be melted, while preventing heat shrinkage of the film, in order to improve a gas-barrier property thereof (Japanese Patent Application Laid-open (KOKAI) No. 8-300549). However, in these conventional methods, although these films are improved in gas-barrier property, retort packaging materials prepared from the laminate using the deposited film are still insufficient in gas-barrier property after the hydrothermal treatments. Further, in the above prior arts, it is not described at all to maintain the good gas-barrier property after the hydrothermal treatments.
In addition, in Japanese Patent Application Laid-open (KOKAI) No. 3-16728, it has been reported that a deposited polyester film having a polyester-based anchor coat layer which undergoes heat history at not lower than the glass transition temperature during the production process of the deposited polyester film, can exhibit a good adhesion between the based polyester film and the deposited layer even when the deposited polyester film is subjected to hydrothermal treatments. However, the heat history specified therein is only an ordinary heat treatment generally used upon vapor deposition steps or film-stretching/heat-treating steps during the production process of deposited films.
The present invention has been made in view of the above conventional problems. An object of the present invention is to provide a gas-barrier film which can exhibit a less deterioration in gas-barrier property upon or after hydrothermal treatments, as well as a laminate including the gas-barrier film.